1. Field
The present invention relates to a method, storage controller, and tape drive for adjusting read heads based on misregistration calculated from servo patterns.
2. Description of the Related Art
Advanced magnetic-tape cartridges will hold multiple Terabytes (TB) of data, where one Terabyte equals 1000 Gigabytes and one Gigabyte equals 1000 Megabytes. The recording densities necessary to achieve this capacity require that the read head elements have nearly or identically the width of the write head elements. Problems can arise in such high density magnetic tape cartridges if the tape medium becomes skewed due to heat or density, which can cause the read heads to read off-track during the write-verify, and this misregistration resulting in either write-verify errors or the inability to write-verify altogether. The write-verify process is actually the reading of the freshly written data during the actual write process.
Servo patterns may be used to determine an extent of misregistration, or the extent to which a read head is reading off the track. In timing-based servo (TBS) systems, recorded servo patterns consist of magnetic transitions with two different azimuthal slopes. Head position is derived from the relative timing of pulses, or dibits, generated by a narrow head reading the servo patterns. TBS patterns also allow the encoding of additional longitudinal position (LPOS) information without affecting the generation of the transversal position error signal (PES). This is obtained by shifting transitions from their nominal pattern position using pulse-position modulation (PPM). A specification for the servo format in current tape drives is provided by the linear tape-open (LTO) format. The complete format for LTO drives of generation 1 (LTO-1) was standardized by the European Computer Manufacturers Association (ECMA) in 2001 as ECMA-319. Additional information on LTO technology, in particular on LTO drives of generations 2 to 4 (LTO-2 to LTO-4), where the servo format was not modified, can be found on the World Wide Web (www) at ultrium.com. Traditionally, the detection of LPOS information bits is based on the observation of the arrival times of the shifted dibit peaks within the servo bursts at the servo reader output.
The timing-based servo (TBS) technology, which was developed specifically for linear tape drives and is also used in all LTO tape drive products, provides the basic structure of a servo frame, consisting of four servo bursts. The signal obtained by reading the servo pattern is used to extract essential servo-channel parameters such as tape velocity, read head y-position, and longitudinal position (LPOS) information, which is encoded by using pulse-position modulation (PPM).